Why are stereo > mono conversions (and reverse) not consistent in levels?

[Guy Burns]

I have a fair understanding of the reasoning behind why mono signals, when converted to stereo, lose 3dB in the L/R channels. What I don't understand is this:

1. I generate a mono 440 Hz tone at -3dB and save it.
2. I use Favorites > Convert to stereo to convert it. The L/R channels are now at -6dB
3. Then I use Favorites > Convert to mono to go back. Result: the level is now at -9dB.

If I kept going long enough, the signal would disappear. Why is that? Surely a Stereo > Mono conversion, followed by a Mono > Stereo conversion should get you back to the original.

I'm using CS6.

[ryclark]

It all depends on what the channel levels are set to in the Favorite which uses the Convert Sample Type effect to make the conversion. When I just tried it the -3dBFS mono came out at -9dB per channel stereo. When converted back to Mono the signal level was -9dB. So something a bit different about our CS6 installations there then.

However you have to remember that when you add two identical audio signals together without any correction the result comes out at 6dB more than the original. If you look in the Convert Sample Type pane (F11 shortcut) you will probably find that the default is set to 50% (ie. -6dB) when doing the conversion. So this is set as a compromise in order to try and maintain Stereo/mono compatibility.  Remembers that most stereo audio does not consist of identical audio on both tracks so a compromise has to be made. Also it is not normal to repeatedly convert backwards and forwards between the two, it is usually just done once. But you can always set up your own conversion levels as a custom Favorite if you want.

[Guy Burns]

The levels in my original post for Step 2 should read -9dB (not -6dB). So that's the same as you found.

I've never used Convert Sample Type Before. Eventually found it under Edit. The defaults are:

Mono to Stereo: Left Mix and Right Mix are set at 100%

Stereo to Mono:  Left Mix and Right Mix are set at 50%

In both cases, all conversions result in the same level. You can do it in a never-ending loop and not lose level. Yet the Favorites that come with the software are set up differently. That's one confusing part. The second confusing part is that when you drag the mono signal into a Multitrack session, the level drops to 71%.

It gets even more confusing: Premiere does it differently (they suggested I come here): a mono signal when dragged to the Timeline is set to 71% for the L/R channel.

100%? 71%? 50%?

There may be some justification for the various settings based on what used to happen in the analogue world mixers – incoming signal is sent to the wiper of the pan pots, the two ends of the posts being the L/R outputs – but that justification certainly hasn't been applied consistently by Adobe.

[ryclark]

Yes we are aware, as end users, that the results between Audition and Premiere aren't consistent unfortunately when moving audio from one to the other.

Some of the justification for the differences may be that Audition has it's origins with the broadcast audio industry where mono/stereo compatibility was required for the listeners and the way that stereo audio was transmitted. However Premiere was always video based and grew up, to begin with, without that broadcasting background.